1. Field of the Invention
This invention relates to a photosensor to be read with the image reading portion of, for example, a facsimile machine, an image reader, a digital type copying machine, etc., and particularly to an image reading device having an image reading portion employing a contact type line sensor, and to an image reading method.
Particularly, the present invention pertains to a photosensor and an image reading device having a correction means for correcting read-out signals on the basis of the standard output, and to an image reading method.
2. Related Background Art
In an image reading device which reads the image information of an original manuscript by irradiating a light on the original manuscript and converting the reflected light to an electrical signal, it is required that an electrical signal always corresponding correctly to the original manuscript should be outputted.
However, in the image reading device of the prior art, due to nonuniformity in the light emission amount of the light source comprising a light emitting device array such as a fluorescent lamp a LED, etc., and nonuniformity in sensitivity and temperature characteristics of the photosensor having photoelectric converting devices, it has been difficult in many cases to output electrical signals correctly corresponding to the original manuscript without photosensor changes over a long period of time.
As a photosensor for photoelectrically converting the reflected light from the original manuscript in such an image reading device, an array of photoelectric converting devices has been employed. Here, it is important that the image information of the original manuscript should be converted correctly to the output electrical signals. Particularly in the case of performing "gradation reading" which outputs three kinds or more of signals corresponding to the shade of the image instead of the "two-value reading" which outputs two kinds of signals of high and low levels, it is necessary to effect output conversion with sufficient precision.
There has already been commercialized, for example, an image reading device which were the contact type image sensor capable of simplifying the constitution due to no need for a reduction lens system because the original manuscript and the sensor portion correspond one to. However, in the device of the prior art, the photoelectric characteristics of the photoelectric converting layer such as A-Si:H film markedly fluctuate depending on the environmental temperature, and since the temperature coefficient can only be made uniform within the array with difficulty, there have been some efforts to output the image information in multiple values. To explain by referring to a readily understandable example, in a case such as continuously reading some photographs having substantially the same density, even if the first photograph may be read correctly, the temperature within the device will be elevated during continuous reading, whereby the sensitivity distribution within the array is changed. Therefore there may sometimes occur the phenomenon that some photographs at the end may be read whiter or darker than the actual original manuscript at specific positions.
Also, in the image reading device of the prior art, there is variance in sensitivity among the photosensitive converting devices of the photoelectric converting device array. This is because the etching depth, film thickness, etc. are varied in the production process for constitution of the above photoelectric converting device array, and it is currently almost impossible to eliminate the variance without reducing the yield. Therefore, it has been difficult for the photoelectric converting device array to convert the image information from the original manuscript to an electrical signal corresponding correctly thereto during output of the electrical signal by scanning for each photoelectric converting device.
Particularly, this point is markedly seen in the case of a sensor which has been lengthened as a contact type image sensor.
For solving such technical tasks, the image reading device with such a constitution as described below has been employed. That is, first, an original manuscript of a patternless white, namely a so called whole whiteness is read, the signals for correction reflecting nonuniformity of sensitivity of the individual photoelectric converting devices are detected and are stored in memory. Next, when reading of the actual manuscript is performed by the above photoelectric converting devices, the signals are corrected using the correction signals, and electrical signals corresponding correctly to the image information of the original manuscript are consequently outputted. An example of such a image reading device is shown in FIG. 1. Here, the outputs of the electrical signals for the white standard are initially received by the reading sensor portion 131 (comprising the photoelectric converting device array) passed through the scanning portion 132 into the memory 133 through the switch 134. Then, on initiation of reading of the original manuscript, The switch 134 is changed over to original to input the signal into the correction circuit 135 to effect correction (calculation such as dividing calculation is done) with the output signals from the memory 133. The outputed signals thus corrected are not effected by the sensitivity variance between the respective photoelectric converting devices and correspond correctly to the image information.
However, in the image reading device of such a system, because the memory for storing the signals for correction therein is required, and also because there may sometimes be differences in uniformity of sensitivity of the photoelectric converting devices caused by temperature, changes over time, etc., it becomes necessary to input the signals for correction immediately before reading for each original manuscript. For this reason, for obtaining the signals for correction, it is required to read the whole white original manuscript every time, or to provide always the whole white portion (generally at the initial head of reading) in the original manuscript, whereby control of the reading device becomes complicated, and also a restriction can occur in the form of the read manuscript.
Accordingly, for providing a device which may effect correction of signals and can be successful commercially, there remains much room for improvement in aspects of performance and cost.
Particularly, these are extremely imminent problems in facsimile machines, etc. for which high speed performance is required.